Problem: Which of these numbers is composite? ${18,\ 31,\ 37,\ 43,\ 83}$
Solution: A composite number is a number that has more than two factors (including 1 and itself). 31, 37, 43, and 83 each have only two factors. The factors of 18 are 1, 2, 3, 6, 9, and 18. Thus, 18 is the composite number.